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Scientists have discovered a new phenomenon on the sun: towering waves that race across its face tsunami-style.

The Solar and Heliospheric Observatory provided a tantalising glimpse of a solar wave about 12 years ago, but it took the three-dimensional view from NASA's STEREO solar probes to nail it.

"It came as a surprise to us when we started seeing these waves expanding," says Joseph Gurman, a solar physicist with NASA's Goddard Space Flight Center .

The waves, which are comprised of plasma, appear at the base of the corona, a couple of thousand kilometres above the surface of the sun. They rise quickly from a central point and spread out in a circular pattern millions of kilometres in circumference.

Scientists using the Solar Terrestrial Relations Observatory (STEREO) confirmed the existence of solar waves in February 2009 when a sunspot erupted, sending a cloud of gas into space and a 100,000-kilometre-high tsunami sprinting across the surface of the sun at about 900,000 kilometres per hour.

The twin STEREO satellites recorded the wave from two positions, giving researchers a three-dimensional view of what had happened.

Plasma burst

The waves are associated with flares and solar storms known as coronal mass ejections, which spew billions of tons of plasma and embedded magnetic fields from the sun's corona into interplanetary space.

Plasma that encounters earth's magnetosphere can trigger powerful geomagnetic storms that can interfere with Global Positioning System radio signals, satellites and other technologies.

Studying how the waves grow and travel should give scientists fresh insights into the sun's magnetic environment, Gurman adds.

"Magnetic field strength tends to be the dominant structure at this level," he says.

Monitoring for waves also should allow solar physicists to pinpoint the source of coronal mass ejections that may be heading toward earth, says Simon Plunkett, with the US Naval Research Laboratory in Washington DC.

"It's not clear from coronagraph observations alone whether a CME is coming toward you or going away from you. It looks almost the same in both directions," says Plunkett.

Scientists had debated whether the tsunami-like feature was actually a magnetic wave propagating through the solar atmosphere, or a footprint of a coronal mass ejection.

Plunkett believes the new study should end that discussion

"It's like a supersonic aircraft during a shock wave ahead of it. The CME is the driving and it's pushing the wave out in front of it."

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